Utilization of Modal Test Techniques for Quality Control of Mass Manufactured Parts

Abstract

Bringing a high tech product to the market as soon as possible has never been so critical. Quality control of critical parts manufactured in large quantities is a problem to solve in many industries ranging from aerospace to automotive. If there are precision parts with very tight dimensional tolerances in the assembly, each and every dimension of every part must be measured. Otherwise parts with dimensions out of tolerances will create more trouble in the later steps of the assembly. Measuring the dimensions of precision parts using high precision coordinate measurement machines (CMM) is time consuming and can be quite cumbersome. Also the initial investment to be made in order to acquire a precise CMM is quite expensive. An alternative solution to this quality assurance (QA) problem is utilizing the vibrational characteristics of the manufactured part. A quality acceptance criterion using those characteristics of the part can be developed. Natural frequencies and mode shapes of the part are the key parameters to be measured when deciding the part to fail or pass. For this purpose, a test setup with the ability of automated modal testing is required. In this study, such a system is designed and developed named as the modal test robot (MTR). The MTR consists of an automated modal test hammer to excite the modes of test part, a mechanism to support the hitting hammer and a heavy granite table to secure the part. By using finite element (FE) analysis, the natural frequency range for the acceptable test part is determined. High fidelity FE analysis models are utilized to ensure good agreement between analysis and test results. Three different parts called the spool; the four arm star and the blade are examined as case studies. In case studies, relationship between part dimension and the natural frequency of the part is calculated by utilizing parametric FE model for each part. Using this relationship, fail or pass criterion is developed according to a specific natural frequency of the part. Natural frequencies of the part are measured on the MTR. Since modal test is carried out by using the MTR, repeatability and coherence of the test results are increased. By this procedure, a technician can make a quick decision whether the measured part passes of fails by just checking the natural frequency. Utilizing the vibrational characteristics as a fail or pass criterion for parts instead of a dimensional measurements is a fast and cheap method that can be employed in QA processes. By implementing this methodology, QA cycle times for manufactured parts reduces 90% and higher number of parts can be controlled in a limited inspection time.